In a wireless network constructed of a plurality of nodes, wireless communication is performed so as to concentrate sensor data detected by a sensor which is mounted on each node in a concentrator. Some nodes perform power saving operation using a battery or environmental power generation (energy harvest) as a power supply. In wireless networks, intermittent communication is generally used to achieve power saving. In order to make most of limited communication resources, communication efficiency needs to be improved, and to improve communication efficiency, it is effective to use multicasting or broadcasting capable of communicating with many nodes at a time and reducing the number of communication times. In order to secure high reliability of the system in particular, it is necessary to efficiently return ACK information (positive response: ACKnowledgment) or NACK information (negative response: Negative ACKnowledgment) in response to transmission of the sensor data.